Method to Support Service Continuity for Multiple Sessions

ABSTRACT

A method to manage call continuity in a network environment including a circuit-switched network and an IP multimedia subsystem (IMS) network is provided. A UE monitors one or more ongoing sessions in an IMS network, and the ongoing sessions contain at least one conference call. The UE detects a network-switching event for the UE to switch from the IMS network to a circuit-switched CS network. The UE then determines the one or more ongoing sessions within the IMS network to be released or transferred to the CS network. Finally, the UE performs a session transfer procedure for transferring the determined sessions.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. §119 from U.S.Provisional Application No. 61/928,815 entitled “Method to Supportservice continuity for multiple sessions,” filed on Jan. 17, 2014, thesubject matter of which is incorporated herein by reference.

TECHNICAL FIELD

The disclosed embodiments relate generally to mobile communicationnetworks, and, more particularly, to support single radio voice callcontinuity when a user equipment (UE) handovers from LTE coverage(E-UTRAN) to UMTS/GSM coverage (UTRAN/GERAN).

BACKGROUND

Long Term Evolution (LTE), commonly marketed as 4G LTE, is a standardfor wireless communication of high-speed data for mobile phones and dataterminals. LTE is based on Global System for Mobile Communications (GSM)and Universal Mobile Telecommunication System (UMTS) technologies thatprovides higher data rate, lower latency and improved system capacity.In LTE systems, an evolved universal terrestrial radio access network(E-UTRAN) includes a plurality of base stations, referred as evolvedNode-Bs (eNBs), communicating with a plurality of mobile stations,referred as user equipments (UEs).

The LTE standard is developed by 3GPP (3^(rd) Generation

Partnership Project) and is specified in its Release 8 document series,with enhancement described in later releases. LTE is the natural upgradepath for carriers with both GSM/UMTS networks and CDMA2000 networks.Along with LTE introduction, 3GPP also standardized Single Radio VoiceCall Continuity (SRVCC) from Release 9 specifications to provideseamless continuity when a UE handovers from LTE coverage (E-UTRAN) toUMTS/GSM coverage (UTRAN/GERAN). With SRVCC, a call anchored in IPmultimedia subsystem (IMS) core can continue to be served by UMTS andGSM networks outside of LTE coverage area.

3GPP Release 9 introduced the basic SRVCC scenario, which is to transfera single call from IMS domain to circuit-switch (CS) domain. Further inlater releases, emergency call, call in different states (e.g., hold,conference), alerting call, and video call are also supported to betransferred from IMS to CS. However, there are still some scenarios thatare not defined clearly when multiple IMS ongoing sessions areestablished. Due to the lack of consistent agreement in 3GPPspecification, the misbehaving UE implementation will result inunexpected call drop and/or un-synchronous call state after the UE campsto a new radio access network or new radio access technology.

To solve the problem, a scheme is disclosed in the present invention tomanage call continuity in a network environment including acircuit-switched network and an IP multimedia subsystem (IMS) networkduring different circumstances.

SUMMARY

A method to manage call continuity in a network environment including acircuit-switched network and an IP multimedia subsystem (IMS) network isprovided. A UE monitors one or more ongoing sessions in an IMS network,and the ongoing sessions contain at least one conference call. The UEdetects a network-switching event for the UE to switch from the IMSnetwork to a circuit-switched CS network. The UE then determines the oneor more ongoing sessions within the IMS network to be released ortransferred to the CS network. Finally, the UE performs a sessiontransfer procedure for transferring the determined sessions.

In one embodiment, the network-switching event includes the UE receivesa Single Radio Voice Call Continuity (SRVCC) Handover command. Anongoing conference call is transferred as an active or held CSmultiparty voice call. If one conference call is transferred, then otherconference calls are released. Each transferred session is mapped to acorresponding CS call control state, auxiliary state, and assigned withcorresponding CS transaction identifiers.

Other embodiments and advantages are described in the detaileddescription below. This summary does not purport to define theinvention. The invention is defined by the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows a mobile communication network havingmultiple radio access networks (RAN) in accordance with one novelaspect.

FIG. 2 is a simplified block diagram of a user equipment (UE) thatsupports embodiments of the present invention.

FIG. 3 illustrates a standardized Single Radio Voice Call Continuity(SRVCC) procedure with enhanced service continuity for multiplesessions.

FIG. 4 illustrates a flow chart of supporting service continuity formultiple sessions in accordance with one novel aspect.

FIG. 5 illustrates one embodiment of session transfer from IMS domain toCS domain.

FIG. 6 illustrates one embodiment of transaction ID assignment from IMSdomain to CS domain.

FIG. 7 is a flow chart of a method of supporting service continuity formultiple sessions in accordance with one novel aspect.

DETAILED DESCRIPTION

Reference will now be made in detail to some embodiments of theinvention, examples of which are illustrated in the accompanyingdrawings.

FIG. 1 schematically shows a mobile communication network 100 havingmultiple radio access networks (RAN) in accordance with one novelaspect. Mobile communication network 100 comprises a first userequipment UE 101 and a second UE 102, an evolved universal terrestrialradio access network (E-UTRAN) 110, a universal terrestrial radio accessnetwork (UTRAN) 120, a serving/packet gateway (S/P GW) 111, a mobilitymanagement entity (MME) 112, an enhanced mobile switching Centre (MSC)121, and an intermediate IP multimedia subsystem (IMS) elements 130. Inthe example of FIG. 1, UE 101 establishes a call with UE 102 via E-UTRAN110 in LTE (EPS/IMS) domain. Later on, when UE 101 moves to anotherlocation, the LTE coverage is no longer available. Instead, onlyUMTS/GSM coverage becomes available. In order to support servicecontinuity, the network will handover UE 101 from E-UTRAN 110 to UTRAN120 where only circuit switch based service is available for voice/videocalls.

For IMS domain calls, there is no restriction in principle. Thepacket-switch based service can support up to M voice calls and N videocalls, where M and N can be any number depending on UE design. Inaddition, the maximum number of a conference call is unlimited. On theother hand, for circuit-switch based service, there are certainrestrictions for calls. For voice calls, CS domain only allows at most 1voice active call+1 voice held call+1 alerting call in parallel. While avoice call can be a single call or a conference call, only oneconference all can be supported, either active or held. The maximumparty number of conference call is 5. For video calls, CS domain videocall cannot be held, and CS domain video call cannot be conference call.

3GPP standardized Single Radio Voice Call Continuity (SRVCC) to provideseamless continuity when a UE handovers from LTE coverage (E-UTRAN) toUMTS/GSM coverage (UTRAN/GERAN). With SRVCC, a call anchored in IMS corecan continue to be served by UMTS and GSM networks outside of LTEcoverage area. 3GPP Release 9 introduced the basic SRVCC scenario, whichis to transfer a single call from IMS domain to circuit-switch (CS)domain. Further in later releases, emergency call, call in differentstates (e.g., hold, conference), alerting call, and video call are alsosupported to be transferred from IMS to CS. For example, thespecification has defined how to handle call transfer with supplementaryfor following cases: 1) keep the call state the same as the originaldomain; 2) if multiple sessions, then the most recent session istransferred as active state, and the second most recent session istransferred as held state.

However, there are still some scenarios that are not defined clearlywhen multiple IMS ongoing sessions are established. Due to the lack ofconsistent agreement in 3GPP specification, the misbehaving UEimplementation will result in unexpected call drop after the UE camps toa new radio access network or new radio access technology. In addition,different understanding of transferred session may result in serioususer experience. In accordance with one novel aspect, call handling isdefined during call transfer for the purpose of synchronization betweenUE and different network entities, and for purpose of smooth userexperience during call transfer. More specifically, the transfer ofconference call and state mapping, and CS auxiliary state and TIassignment are defined when UE handovers from LTE coverage to UMTS/GSMcoverage with service continuity.

FIG. 2 is a simplified block diagram of a user equipment (UE) 201 thatsupports embodiments of the present invention. UE 201 has RF transceivermodule 202, coupled with antenna 211, receives RF signals from antenna211, converts them to baseband signals and sends them to processor 203.RF transceiver 202 also converts received baseband signals from theprocessor 203, converts them to RF signals, and sends out to antenna211. Processor 203 processes the received baseband signals and invokesdifferent functional modules to perform features in the UE. Memory 204stores program instructions and data to control the operations of theUE.

FIG. 2 further illustrates four functional modules 221 to 224 in UE 201that carry out embodiments of the current invention. The functionalmodules may be implemented by hardware, firmware, software, or anycombination thereof. The function modules, when executed by processors203 (e.g., via executing program codes 205), for example, allow UE 201to properly detect handover event and perform handover from IMS domainto CS domain. In one example, a call management module 221 that managesvarious configuration related to an established radio connection andongoing sessions, a measurement module 222 performs radio signalmeasurement and thereby monitoring the radio connection, anevent-detection module 223 detects various event such as receiving ahandover command, and a handover module 224 that implements variousfeatures of handover and call transfer supporting voice call servicecontinuity for multiple sessions.

FIG. 3 illustrates a standardized Single Radio Voice Call Continuity(SRVCC) procedure with enhanced service continuity for multiplesessions. Mobile communication network 300 comprises UE 301, E-UTRAN302, MME 303, MSC server 304, target MSC 305, target SGSN 306, and IMS307. In step 311, a UE 301 establishes a voice over IP (VOIP) sessionover the IMS core on E-UTRAN radio access in an LTE cell. In step 312,UE 301 performs radio signal measurements and sends a measurement reportto E-UTRAN 302. For example, the measurement report indicates that theradio signal strength or quality of the LTE cell is below a predefinedthreshold. This could happen when UE 301 moves out of the LTE coveragearea. Based on the measurement report, the network makes handoverdecision in step 313. For example, the network decides to handover UE301 from E-UTRAN to UTRAN/GERAN, which supports CS-based call only. Instep 314, E-UTRAN 302 sends out a handover request to mobilitymanagement entity MME 303, which starts handover preparation. In step321, MME 303 performs bearer splitting. In step 322, CS-HO preparationis performed by MME 302, MSC server 304, and target MSC 305. In step323, IMS session continuity procedure is performs by MSC server 304,target MSC 305, target SGSN 306, and IMS 307. If non-voice bearerexists, in step 324, PS-HO procedure begins among MME 303, MSC server304, target MSC 305, and target SGSN 306. After handover preparation,MME 303 sends a HO command to E-UTRAN 302 in step 331. In step 332,E-UTRAN 302 forwards the HO command to UE 301. After receiving the HOcommand, handover execution is then started. In step 341, UE 101 movesto UTRAN/GERAN from E-UTRAN. In step 342, circuit-switched CS handoveris completed. In step 343, MSC server 304 sends a PS to CS completemessage to MME 303, which optionally sends a TMSI reallocation messageto UE 301 in step 344. In step 345, MSC server 304 performs locationupdate. Finally, in step 346, packet-switched PS handover is completed.

SRVCC is a level of functionality that is required within LTE systems toenable the packet domain calls on LTE to be handover to legacy circuitswitched voice systems such as GSM, UMTS, and CDMA in a seamless manner.SRVCC is a scheme that enables inter radio access technology (inter-RAT)handover as well as handover from packet data to circuit switched datavoice calls. By using SRVCC, operators are able to make the handoverswhile maintaining existing quality of service (QoS) and ensure that callcontinuity meets the critical requirements for emergency calls. TheSRVCC implementation controls the transfer of calls in both directions.Handover from LTE to the legacy network is required when the user movesout of the LTE coverage area. Using SRVCC, the handover is undertaken intwo stages. A first stage is the radio access technology (RAT) transfer.A second stage is session transfer. The session transfer is required tomove the access control and voice media anchoring from the EvolvedPacket Core (EPC) of the packet switched LTE network to the legacycircuit switched network.

The SRVCC handling is performed in step 341. The current SRVCC handling,however, has not defined call transfer involving multiple ongoingsessions and conference calls. If UE 301 has established multipleongoing sessions in IMS domain, and if the multiple ongoing sessionsinclude conference call(s), then the UE shall follow specific rulesduring the call transfer. The UE needs to determine 1) which sessionshould be transferred or released, 2) how to map the CS call state andthe auxiliary state for the transferred session; and 3) how to set theCS transaction identifier (TI) assignment.

FIG. 4 illustrates a flow chart of supporting service continuity formultiple sessions in accordance with one novel aspect. In step 401, a UEreceives a SRVCC handover command from the network. The handover commandis for the UE to handover from E-UTRAN to UTRAN/GERAN, e.g., when the UEmoves out of the LTE coverage area. In step 402, the UE determineswhether the SRVCC handover is successful. If not, then the UE fails tomaintain the voice call continuity (step 403). If yes, then the UEcontinues to handle SRVCC. In step 411, the UE handles session transferfor its multiple ongoing sessions from PS domain to CS domain. The UEdetermines which session(s) to be transferred to CS domain and whichsession(s) to be released in CS domain. In step 412, the UE sets thecircuit-switched call state and auxiliary state for the transferredsession(s). In step 413, the UE sets the circuit-switched transactionID.

FIG. 5 illustrates one embodiment of session transfer from IMS domain toCS domain. In IMS domain, a UE may have established multiple ongoingsessions, and each session having different call state. A call can be asingle voice call, a conference call, or a video call. A call can beactive, held, or alerting. In CS domain, a call state includes null,alerting, call delivered, call active (A), and an auxiliary stateincludes held and multiparty (MPTY). The general SRVCC principles remainthe same: 1) keep the call state the same as the original domain; 2) ifmultiple sessions, then the most recent session is transferred as activestate, and the second most recent session is transferred as held state.However, special scenarios occur when multiple ongoing sessions withconference call(s) are involved. In general, if only one ongoing sessionwith active or inactive speech media component exists, and the ongoingsession with active or inactive speech media component is a conferencecall, then the UE shall replace the speech media component of theongoing session with the newly established active or held CS multipartyvoice call. If more than one ongoing session with active or inactivespeech media component exist, and at least one of the ongoing sessionswith active or inactive speech media component is a conference call,then the UE shall select and replace the speech media component of oneof the ongoing sessions with the newly established active or held CSmultiparty voice call. The selection is determined based on when theactive or inactive speech media component was made, e.g., mostlyrecently or second most recently. If one conference session is to bereplaced and transferred to CS domain, then all other conferencesessions shall be released.

Table 500 illustrates different scenarios for transfer of conferencecall(s) and state mapping. In a first scenario, the most recently activecall is a conference call, and the second most recently active call is anon-conference call. When transferring to CS domain, the most recentlyactive call is transferred to become an active multi-party (A MPTY)call, and the second most recently active call is transferred to becomea held call. This is true regardless of any inactive calls. In a secondscenario, the most recently active call is a non-conference call, andthe second most recently active call is a conference call. Whentransferring to CS domain, the most recently active call is transferredto become an active (A) call, and the second most recently active callis transferred to become a held MPTY call. This is true regardless ofany inactive calls. In a third scenario, the most recently active callis a conference call, there is no other active call, and the mostrecently inactive call is a non-conference call. When transferring to CSdomain, the most recently active call is transferred to become an activemulti-party (A MPTY) call, and the most recently inactive call istransferred to become a held call. This is true regardless of any otherinactive calls. In a fourth scenario, the most recently active call is anon-conference call, there is no other active call, and the mostrecently inactive call is a conference call. When transferring to CSdomain, the most recently active call is transferred to become an active(A) call, and the most recently inactive call is transferred to become aheld MPTY call. This is true regardless of any other inactive calls.Finally, in a fifth scenario, there are no active calls. The mostrecently inactive call is a conference call, which is transferred to CSdomain as a held MPTY call, regardless of any other inactive calls.

The 3GPP LTE specification also defines CS call control state mappingfor SRVCC after handover to CS domain. When a session is transferred toa CS active call, the UE shall move to Call Active (U10) state definedin 3GPP TS24.008. When a session is transferred to a CS held call, theUE shall move to Call Active (U10) state defined in 3GPP TS24.008 andCall Held Auxiliary State defined in 3GPP TS24.083. When a session istransferred to a CS MPTY call, the UE shall move to Call Active (U10)state defined in 3GPP TS24.008 and Call in MPTY Auxiliary State definedin 3GPP TS24.084.

When session transfer is performed from IMS domain to CS domain, only atmost two sessions will be transferred. The UE shall associate eachtransferred session with a transaction identifier (TI) as following: Ifthe active IMS ongoing session is a conference call and has beentransferred to CS active (A) call, the UE shall associate theparticipants in the conference session with CS call with transactionidentifier 0, 2, 3, 4, 5. If two sessions are transferred, the firsttransferred session is a non-conference call and the second transferredsession is a conference call, then the UE shall associate the firsttransferred session with CS call with transaction identifier 0, andassociate the participants in the conference session with CS call withtransaction identifier 1, 3, 4, 5, 6.

FIG. 6 illustrates one embodiment of transaction ID assignment from IMSdomain to CS domain. In the left diagram of FIG. 6, a conference call inIMS domain is transferred to CS domain as a CS active multi-party call.In such a case, the UE shall associate the participants in theconference session (multi-party) with CS call with transactionidentifiers 0, 2, 3, 4, 5. In the right diagram of FIG. 6, a singlevoice call is transferred to CS domain as an active call, and aconference call in IMS domain is transferred to CS domain as a CS heldmulti-party call. In such a case, the UE shall associated the singlevoice call with CS call with transaction identifier 0, and associate theparticipants in the conference session (multi-party) with CS call withtransaction identifiers 1, 3, 4, 5, 6. This way, the TI assignment isconsistent with legacy CS domain voice calls.

Alerting session is also supported by IMS domain and needs to behandled. The alerting session may be released or transferred to CSdomain. When releasing the alerting session, UE may send a SIP 480(Temporary Unavailable) response to reject the SIP INVITE request. Inthe current 3GPP restriction, when there is 1A+1Held+1alerting, thealerting session will be released. However, it is an available situationin CS domain, and the alerting session can be transferred to CS domain.

FIG. 7 is a flow chart of a method of supporting service continuity formultiple sessions in accordance with one novel aspect. In step 701, a UEmonitors one or more ongoing sessions in an IMS network, and the ongoingsessions contain at least one conference call. In step 702, the UEdetects a network-switching event for the UE to switch from the IMSnetwork to a circuit-switched CS network. In step 703, the UE determinesthe one or more ongoing sessions within the IMS network to be releasedor transferred to the CS network. In step 704, the UE performs a sessiontransfer procedure for transferring the determined sessions.

Although the present invention has been described in connection withcertain specific embodiments for instructional purposes, the presentinvention is not limited thereto. Accordingly, various modifications,adaptations, and combinations of various features of the describedembodiments can be practiced without departing from the scope of theinvention as set forth in the claims.

What is claimed is:
 1. A method, comprising: (a) monitoring one or moreongoing sessions in an IP multimedia subsystem (IMS) network by a userequipment (UE), wherein the ongoing IMS sessions contain at least oneconference call; (b) detecting a network-switching event for the UE toswitch from the IMS network to another circuit-switched (CS) network;(c) determining the one or more ongoing sessions within the IMS networkto be released or transferred to the CS network; and (d) performing asession transfer procedure for transferring the determined sessions. 2.The method of claim 1, wherein the network-switching event includes theUE receives a Single Radio Voice Call Continuity (SRVCC) Handovercommand.
 3. The method of claim 1, wherein one ongoing session with anactive speech media component is a conference call, and wherein (c)involves: replacing the speech media component of the ongoing conferencecall with a newly established active CS multiparty voice call.
 4. Themethod of claim 1, wherein two or more ongoing sessions with an activespeech media component exist, wherein the most recently active sessionis a conference call, and wherein (c) involves: replacing the speechmedia component of the most recently active conference call with a newlyestablished active CS multiparty voice call; releasing all other IMSongoing session that is a conference call; and replacing the speechmedia component of the second most recently active session with a newlyestablished held CS voice call.
 5. The method of claim 1, wherein two ormore ongoing sessions with an active speech media component exist,wherein the second most recently active session is a conference call,and wherein (c) involves: replacing the speech media component of themost recently active ongoing session with a newly established active CSvoice call; and replacing the speech media component of the second mostrecently active conference call with a newly established held CSmultiparty voice call.
 6. The method of claim 1, wherein no ongoingsession with an active speech media exists, wherein one ongoing sessionwith an inactive speech media component is a conference call, andwherein (c) involves: replacing the speech media component of the mostrecently inactive conference call with a newly established held CSmultiparty voice call.
 7. The method of claim 1, wherein one ongoingsession with an active speech media component exist and one ongoingsession with an inactive speech media component is a conference call,and wherein (c) involves: replacing the speech media component of themost recently active session with a newly established active CS voicecall; and replacing the speech media component of the most recentlyinactive conference call with a newly established held CS multipartyvoice call.
 8. The method of claim 1, wherein the session transferprocedure (d) involves mapping each transferred session to acorresponding CS call control state, auxiliary state, and transactionidentifier (TI) assignment.
 9. The method of claim 1, wherein an activeIMS ongoing session is a conference call and has been transferred as anactive CS multiparty voice call, and wherein the UE associates theparticipants in the conference call session with transaction identifiers0, 2, 3, 4, and
 5. 10. The method of claim 1, wherein a first IMSsession has been transferred as an active CS voice call, wherein asecond IMS session has been transferred as a held CS multiparty voicecall, wherein the UE associates the first session with transactionidentifier 0, and wherein the UE associates the participants in thesecond session with transaction identifiers 1, 3, 4, 5, and
 6. 11. Amethod, comprising: (a) monitoring one or more ongoing sessions in an IPmultimedia subsystem (IMS) network by a user equipment (UE), wherein theongoing IMS sessions contain at least one active session, one inactivesession, and one alerting session; (b) detecting a network-switchingevent for the UE to switch from the IMS network to anothercircuit-switched (CS) network; (c) determining the one or more ongoingsessions within the IMS network to be released or transferred to the CSnetwork; and (d) performing a session transfer procedure fortransferring the determined sessions.
 12. The method of claim 11,wherein the network-switching event includes the UE receives a SingleRadio Voice Call Continuity (SRVCC) Handover command.
 13. The method ofclaim 11, wherein one ongoing session with an active speech mediacomponent is a conference call, and wherein (c) involves: replacing thespeech media component of the most recently active ongoing conferencecall with a newly established active CS voice call; replacing the speechmedia component of the most recently inactive ongoing conference callwith a newly established held CS voice call; and handling the alertingsession.
 14. The method of claim 13, wherein the handling the alertingsession involves releasing the alerting session or transfer the alertingsession to CS domain.
 15. The method of claim 14, wherein the releasingthe alerting session further involves sending a Temporarily UnavailableSIP response to reject an SIP request.
 16. A user equipment (UE),comprising: a measurement module that monitors one or more ongoingsessions in an IP multimedia subsystem (IMS) network, wherein theongoing IMS sessions contain at least one conference call; an eventdetection module that detects a network-switching event for the UE toswitch from the IMS network to another circuit-switched (CS) network; acall management module that determines the one or more ongoing sessionswithin the IMS network to be released or transferred to the CS network;and a handover module that performs a session transfer procedure fortransferring the determined sessions.
 17. The UE of claim 16, whereinthe network-switching event includes the UE receives a Single RadioVoice Call Continuity (SRVCC) Handover command.
 18. The UE of claim 16,wherein the UE replaces a speech media component of a most recentlyactive conference call with a newly established active CS multipartyvoice call, and releases all other IMS ongoing session that is aconference call.
 19. The UE of claim 16, wherein the UE replaces aspeech media component of a most recently inactive conference call or asecond most recently active conference call with a newly establishedheld CS multiparty voice call.
 20. The UE of claim 16, wherein the UEmaps each transferred session to a corresponding CS call control state,auxiliary state, and transaction identifier (TI) assignment.
 21. The UEof claim 16, wherein the UE handles an alerting session by releasing thealerting session or transferring the alerting session to CS domain.